Rinnai gas heater loud noise on ignition

It may be, Rinnai suggests a maximum length of about 13 equivalent feet of venting including elbows. Add 6 feet for every 90 degree elbow. If the vent is longer than this it is taxing the combustion fan motor and it will start giving you error codes and will not ignite. Remove the trim panel from behind the heater and find the flexable air intake pipe. Temporally remove it from a fitting and test the unit a few times to see if the unit starts quieter. This procedure will be taking room air into the heater for combustion and circumventing the direct vent feature of the heater, not a fix. Have you had the gas pressure tested? It gould be that! You may wish to consider relocating the heater and fire your installation contractor for putting you in this predicament, not Rinnai.